A Systematic Evaluation of Time-Temperature Indicators for Use As Consumer Tags

A Systematic Evaluation of Time-Temperature Indicators for Use as Consumer Tags

MAUREEN SHERLOCK', BIN FU, PETROS S. TAOUKIS2, and THEODORE P. LABUZA*

Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Ave., St. Paul, Minnesota 55108Downloaded from http://meridian.allenpress.com/jfp/article-pdf/54/11/885/1662368/0362-028x-54_11_885.pdf by guest on 02 October 2021 (Received for publication June 14, 1991)

ABSTRACT Ideally, what is needed is a cost effective way to individually monitor the storage conditions of the products The applicability of single end point, consumer readable throughout distribution and indicate their remaining shelf Time-Temperature Indicators (TTIs) as monitors of the end of shelf life of refrigerated food products was examined. Two types life. Such a system could lead to effective quality control of of consumer TTIs, an enzymatic and a polymer based, were tested the distribution and stock rotation of perishable foods and under isothermal and nonisothermal conditions. Their temperature could further give the consumer meaningful information dependence followed the Arrhenius equation (activation energy about product freshness. Time-Temperature Indicators (TTIs) 11 to 24 kcal/mol), and except for one tag model, the response are a step towards this direction. A Time-Temperature obtained under variable temperature conditions agreed well with Indicator is a small, inexpensive device that shows an that predicted from the Arrhenius equation. Evaluation showed easily measurable, time-temperature dependent, irreversible satisfactory visual recognition of the end point as compared to an change that can be correlated to quality changes of a food instrumental measurement. Consumer TTIs can be reliably used product that undergoes the same temperature distribution. as end of shelf-life indicators for foods with a similar activation energy for deterioration. Early types of TTIs (2) showed considerable technical inefficiencies and unreliability. However, it has been shown that new generation TTIs reliably correlate to changes of Storage conditions are frequently less than ideal in the tested perishable foods (77). distribution system for perishable food products, and low Recent work by Taoukis and Labuza (8-10) has shown storage temperature is vital to the quality and safety of such how continuous response TTIs can be applied as quality products (6). The need for a tightly monitored distribution monitors for distribution control and stock rotation man as well as for an alternative for open-date labeling has been agement, and how single end point TTIs (consumer tags), repeatedly emphasized, because an open-date label on a readable by a consumer, can be applied as active freshness food package does not tell anything about the actual tem indicators, instead of, or in conjunction with open-date labels. The use of consumer tags on perishable foods, not perature history of the food product. There are many only gives the product a value-added profile, but also may different kinds of open dates currently being used in the quell the concerns of consumers who want high quality food industry (4), but even the most appropriate open date, convenience foods without worrying about the freshness of when used on a food package, falls short from giving any foods. In addition to the emerging refrigerated meals mar information on the actual distribution conditions and thus ket, other food product categories, including dairy products, the real quality state of individual products. fresh meat, fish, and poultry are likely targets for consumer The Lempert Report (7) estimates that United States type TTIs. However, there is a question of whether or not sales of ready-to-eat chilled entrees could top $1.5 billion the tags are consumer friendly, i.e., are consumers willing in three years. This recent proliferation of freshly prepared, to read these indicators and determine whether or not packaged, refrigerated meals makes this study crucial to the temperature abuse has occurred (7)1 safety and quality of this product category. This product For a successful application of these TTIs, they have to category is very dependent upon the maintenance of refrig fulfill some unique requirements with regards to their temp eration temperature throughout the entire distribution chain erature behavior, reliability, and ease and accuracy with as spoilage or pathogen growth is a function of both time which they are interpreted by the consumer. The objective and temperature. of this study was to conduct an assessment of two different types of consumer TTIs. This included developing an objective instrumental method of color change measure ' International Flavors & Fragrance Co., NJ. 2 Technology of Agriculture Products, Ministry of Agriculture, Lybovrissi,ment and end point determination, testing the reproducibil Greece ity of the response, developing a model of the temperature

JOURNAL OF FOOD PROTECTION, VOL. 54, NOVEMBER 1991 886 EVALUATION OF TIME-TEMPERATURE INDICATORS behavior using the Arrhenius equation if applicable, testing the Arrhenius behavior under variable temperature conditions, the extent to which the response under non isothermal nonisothermal storage experiments were conducted for all TTIs conditions could be reliably predicted by the developed (five samples per experiment). An environmental chamber with a computerized controller was used to generate a sinusoidal models, and finally examining the accuracy with which an temperature variation, fluctuating between 2 and 18°C (mean observer can distinguish the end point as compared to an temperature 10°C), within a 4 h period. The measured end point instrument. under these conditions was compared to the one predicted from the Arrhenius model using the integration procedure of Taoukis MATERIALS AND METHODS and Labuza (9).

Description of TTIs Visual evaluation Two types of consumer TTIs were tested. TTI type I is based A panel of between 12 to 15 untrained judges consisting of on the solid state polymerization of a colorless, thinly coated faculty and students in the Department of Food Science and diacetylenic monomer to a dark blue colored polymer. The chang Nutrition at the University of Minnesota participated in the visual ing part is set in the center of a colored reference ring. The end evaluation sessions that took place on 6 testing days. They were point is reached when the center becomes the same as or just chosen based on their willingness to participate and availability, Downloaded from http://meridian.allenpress.com/jfp/article-pdf/54/11/885/1662368/0362-028x-54_11_885.pdf by guest on 02 October 2021 darker than the outer reference ring. Two models of this type were and were rewarded for their participation. At an appropriate time tested (Fresh Check A40 and A20, Lifelines, Technology Morris before each evaluation session, time-temperature devices were Plains, NJ). taken out of the freezer and placed in incubators at 25 °C. Judges TTI type II is based on the enzymatic hydrolysis of a lipid were asked to return to the testing area on the average of about substrate that causes a change in pH expressed in a color change every hour to evaluate the tags. of an appropriate mixture of indicators. The color change is from A paired comparison test, based on visual observation of dark green to yellow and can be compared to a printed reference. whether the indicator color was lighter, the same as or darker than Two models were tested (TTI Monitors 3014 and 4014, I-Point, the reference value, was run with the null hypothesis that the Malmoe, Sweden). The TTI types tested had end points ranging indicator was lighter in color than the reference, and the alterna from 1 to 5 weeks at 4°C. tive hypothesis being that the indicator color was the same or As visual color is the criterion for the user, a tristimulus darker than the reference value for the TTI type I Models A 40 colorimeter was used for measuring the response (3). A Minolta and A20. For TTI type II Models 3014 and 4014, the same paired Chromameter CR-221 with a 3-mm measuring head and a CIE comparison test was performed with the null hypothesis stating Illuminant C was used to measure the L*a*b* values. From this, that the indicator color was the same as or darker than a printed 2 2 2 the total color difference AE=\AL + Aa + Ab and the chroma reference color and the alternative hypothesis being that the ind 2 2 difference AC =VAa + Ab over time could be calculated where icator color was the same as or lighter than the printed reference. each A refers to the difference in value from the standard refer The number of judges to significantly determine the end point at ence printed on the indicator. Thus, when AE and/or AC reach the the 95% confidence level requires 10 of 12, 11 of 13, 12 of 14, minimum difference, the changing color part is closest to the or 12 of 15 panelists with the same response (5). reference value and thus the end of shelf life is reached. The reference values are listed in Table 1 for each type of TTIs. RESULTS AND DISCUSSION

TABLE 1. L*a*b* reference values. Constant temperature studies Type I. Fig. 1 shows an example of the AE value vs. TTI L a b time for 12 samples evaluated at 4 and 21°C. Theoretically, A40 63.54 16.92 -3.66 the end point would be at AE=0 for a perfect color match, A20 64.52 0.78 -15.35 but the changing center color does not exactly match the 3014 57.21 -8.30 41.71 printed reference ring color at any time because of the way 4014 57.55 -8.24 39.65 the colors are printed. There was very little variability between samples at any given temperature. The end points Constant temperature studies determined from the individual TTI tags were practically The first set of experiments involved storage of 12 type I tags identical, showing a coefficient of variation (CV) of equal (A20 & A40) at five constant temperatures of 4, 7, 13, 21, and to or less than 2% for both models of TTI type I (Table 2). 31°C. For type II, five tags of Model 3014 and six tags of Model The average determined instrumental end points and 4014 were stored at 4, 7, 13, and 25°C. Measurements were taken at appropriate intervals until the colors were far beyond their end standard deviations at the different temperatures and the points. The time at which the resultant curve of AE or AC showed Arrhenius parameters are also shown in Table 2. Fig. 2 a minimum value was used as the instrumental end point. Statis shows the Arrhenius plot of the results and indicates a very tical analysis of the variability in the end points was conducted good fit (r2> 0.99), validating the model. The determined from the individual samples at each temperature. The inverse of activation energy of about 20 kcal/mol is similar to that the obtained end point times at different temperatures were plotted found for polymer based continuous response tags by in a semilog plot vs. the inverse absolute temperature (T), to test Taoukis and Labuza (8,9). the applicability of the Arrhenius model. Activation energy (EA) was obtained from its slope (slope = - E /R). A Type II. For TTI type II, when AE was plotted vs. time Variable temperature studies (Fig. 3a), a distinct minimum occurred, but a large variabil After modeling the temperature dependence of the TTI based ity between replicates was observed. This variability was on the constant temperature experiments, one should be able to partly due to an inconsistency in the change of the L value predict the behavior at any temperature condition, which is the with time. On the other hand, the chroma parameters a and premise of a reliable TTI. To test if there are any deviations from b showed a consistent and less variable change. Thus, the

JOURNAL OF FOOD PROTECTION, VOL. 54, NOVEMBER 1991 EVALUATION OF TIME-TEMPERATURE INDICATORS 887 TABLE 2. End point times for TTI type I*. chroma difference AC - lAa2 + Ab2 was used as the end point criterion in Fig. 3b. Both AE and AC plots gave the Temperature Model A20 Model A40 same average end point, but the variability between samples (°C) End point (h) CV (%) End point (h) CV (%) was much smaller when the AC was used. The coefficient 4 763.9 ± 15.3 2.0 263.9 ±4.4 1.7 of variation for Model 4014 was a little larger than the type 7 540.5 ±1.5 1.5 160.5 + 2.3 1.4 I tags (about 5%; Table 3), but it was still acceptable. 13 282.9 ± 0.9 0.9 85.3 ±1.0 1.2 However, it seems that the CV for Model 3014 is tempera 21 109.8 ± 2.2 2.0 33.3 ± 0.7 2.1 ture dependent, the lower the temperature, the larger the 31 33.2 ±2.1 2.1 10.7 ± 0.2 1.9 variability. Table 3 also shows the average determined end points (+sd) as well as the Arrhenius parameters deter Arrhenius parameters (±95% confidence intervals): mined from the different temperature values. A good fit Ink 28.6 ± 2.9 29.9 ± 2.3 o was found for Model 4014 (r2 > 0.99), as seen in Fig. 4, EA (kcal/mol) 19.4 + 1.7 19.5 + 1.3 r2 0.998 0.999 with an activation energy of 24.3 kcal/mol. Poor fit to the Arrhenius model (see Fig. 4) and large variability among

*n=12. tags make Model 3014 unacceptable for any practical use. Downloaded from http://meridian.allenpress.com/jfp/article-pdf/54/11/885/1662368/0362-028x-54_11_885.pdf by guest on 02 October 2021 This poor fit may be due to the fact that at the higher temperature (lower 1/T), the enzyme is at or beyond its 40 temperature optimum so the actual rate is less than pre dicted from a In k vs. 1/T plot. A40at21°C 30

U 20- „ I i I A20 at 4 °C

10 il

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0 200 400 600 800 1000 10 15 Time (hr) (a) Time (hr) Figure 1. Plot of total color difference AE vs. time for TTI type I Model A40 at 21°C and Model A20 at 4°C.

model A40

O .01 :

0 5 10 15 (b) Time (hr) .001 3-2 3.3 3.4 3.5 3.6 3.7 Figure 3. (a) Plot of total color difference AE vs. time for TTI type II (Model 1/TxlO 3(K_1 ) 4014) at 25°C; (b) Plot of chroma difference AC vs. time for TTI type II (Model Figure 2. Arrhenius plot for TTI type I (Models A20 and A40). 4014) at 25°C.

JOURNAL OF FOOD PROTECTION, VOL. 54, NOVEMBER 1991 888 EVALUATION OF TIME-TEMPERATURE INDICATORS

poor Arrhenius behavior of this particular model. The Model 4014 fell about in the middle of the predicted confidence range. The average error for Model 4014 was within 5% of the predicted values which is probably ac ceptable. The huge difference between the predicted and the experimental value for Model 3014, mainly due to the lack of fit of the Arrhenius model and large sample vari ability, makes this model not applicable for a consumer tag.

TABLE 4. Comparison of observed vs. predicted end point times for sinusoidal temperature fluctuation between 2 and 18°C with a 4 h period.

TTI Observed* Predicted** Downloaded from http://meridian.allenpress.com/jfp/article-pdf/54/11/885/1662368/0362-028x-54_11_885.pdf by guest on 02 October 2021 end point (h) end point (h) %A *** A20 301.0 ±4.7 306.5 1.8 1/TxlO 3(K_1 ) (295.5 - 317.0)

TABLE 3. End point times for TTI type I*. A40 96.9 ± 0.8 96.2 0.7 (93.5 - 98.8) Temperature Model 3014* Model 4014* (°C) End point (h) CV (%) End point (h) CV (%) 3014 81.9 ±7.2 106.5 23.1 (88.9 - 114.7) 4 209.2 ± 29.2 13.9 248.5 ± 10.0 4.0 7 139.1 ± 13.9 10.0 122.6 ±5.1 5.1 4014 59.2 ±3.0 62.4 5.1 13 72.3 ± 1.8 2.5 51.6 ± 2.6 5.0 (53.7 - 70.8) 25 46.6 ± 0.8 1.7 10.3 + 0.5 4.8 *+sd, n=5. Arrhenius parameters (+95% confidence intervals): **95% confidence intervals listed in brackets. ***lobserved-predictedl/predicted *100. Inko 15.5 ± 117.7 38.6 + 9.9 EA° 11.4+10.0 24.3 ±5.6 r2 0.923 0.994 Correlation between visual evaluation and instrumental analysis *n=5; *n=6. The visual evaluations for five tags of each of the both types as compared to the instrumental end point for that tag Variable temperature studies are summarized in Table 5. Fig. 5 shows the time course To evaluate the reliability of the tags under nonisother- for the A40 and 4014 tags. As noted before, for type I tags, mal conditions, the results of tags exposed to sinusoidally the visually determined end point is the first point at which changing temperature conditions were compared to the the significant number of panelists characterize the TTI the value predicted using the Arrhenius model based on a same color as or just darker than the reference as required formula similar to the one developed by Taoukis and for a 95% significance level. For type II tags the visually Labuza (9) for a sine wave distribution: determined end point is the first point at which the signifi E.a -i cant number of panelists characterize the TTI the same as Ll RT (T + a ) I or darker than a printed reference as required for a 95% [ v / m m o —• significance level. where 0sm is the predicted end point for the fluctuation, 0 is the end point time at the mean temperature of the TABLE 5. Comparison of visually evaluated vs. instrumentally fluctuation (Tm), ao is the temperature amplitude, and I (X) measured end point for five TTI tags of each type. is a modified zero order Bessel function of the function in the brackets. Table 4 shows the results of comparing the Type I A40 Type I A20 actual vs. the predicted end points. Visual (h) Instrument (h) %A Visual (h) Instrument (h) %A For type I tags, the experimentally observed and the Tag 1 6.3 6.1 3.3 10.9 10.9 0 predicted end point times had a small difference with about Tag 2 6.4 6.4 0 11.0 10.9 1.0 an error of 1-2% and were not different at the 95% Tag 3 6.4 6.3 1.6 11.4 11.2 1.8 confidence level. Taoukis and Labuza (9) discussed that Tag 4 6.1 6.3 3.3 11.1 10.8 2.8 such a small error was not deemed to be significant with Tag 5 5.8 6.2 6.5 10.7 10.8 0.9 respect to prediction of actual food shelf life. Type II 3014 Type II 4014 However, for the type II tags, the error was about 5% Visual (h) Instrument (h) %A Visual (h) Instrument (h) %A for Model 4014 and 23% for Model 3014. As noted for Tag 1 49.9 48.0 4.0 10.0 10.5 4.8 Model 3014, the difference in observed vs. predicted was Tag 2 48.0 46.0 4.4 10.0 10.0 0 not statistically different at the 95% confidence level, but Tag 3 48.5 48.5 0 9.5 10.0 5 this was just borderline at the lower time limit. Besides, the Tag 4 50.5 50.5 0 9.5 9.5 0 confidence range was wide due to the variability and the Tag 5 46.5 49.0 5.1 11.5 9.5 21.1

JOURNAL OF FOOD PROTECTION, VOL. 54, NOVEMBER 1991 EVALUATION OF TIME-TEMPERATURE INDICATORS 889 use as active consumer shelf life labels is reported. The tags were found to follow Arrhenius kinetics except for one I- Point model. Arrhenius kinetics could be applied satisfac torily in predicting the end point under variable temperature conditions for type I (both Model A20 and Model A40) and for type II (Model 4014 only) tags. An instrumental method of measurement was developed for the determination of the end point time that allows accurate setting of the TTI specifications. The visual characteristics of the TTIs were specific enough to allow 95% of the consumers to visually recognize the end point of the TTIs within 5% of the instrumental end point, which is of critical importance. For a consumer tag, where there is only one end point, the requirements for agreement between the activation Time (hr)

energy of the food and TTIs is very strict (+2 kcal/mol) for Downloaded from http://meridian.allenpress.com/jfp/article-pdf/54/11/885/1662368/0362-028x-54_11_885.pdf by guest on 02 October 2021 successful application in predicting the end point of the food (9). However, most refrigerated foods, including those packaged with modified atmosphere, have an activa tion energy of 20-25 kcal/mol and a shelf life ranging between one week to several months at 4°C. Thus, TTI type I (both Model A20 and Model A40) and TTI type II (Model 4014) can meet the requirements on both activation s- energy and shelf life.

ACKNOWLEDGMENT

isi Published as paper #19,138 of the contribution series of the Minne sota Agricultural Experiment Station based on research conducted under Project No. 18-78. This study was also supported in part by the I-Point Co., Malmoe Sweden and the 3M Co., St. Paul, MN.

5 10 REFERENCES (b) Time (hr) 1. Anonymous. 1989. Marketing analysis, issues and trends: Doing a Figure 5. fresh check. June 28, 1989. The Lempert Report, vol. 5, No. 4. (a) Visual evaluation results of TTI type I (Model A40); 2. Byrne, C. H. 1976. Temperature indicators - the state of the art. (b) Visual evaluation results of TTI type II (Model 4014). Food Technol. 30(6):66-68. 3. Francis, F. J., and F. M. Clydesdale. 1975. Food colorimetry: theory and applications. AVI Publishing Co., Westport, CT. The difference in time at which all panelists judged the 4. Institute of Food Technologists. 1981. Open shelf-life dating of tag as different or lighter compared to the time all judged food: A scientific status summary. Food Technol. 35(2):89-96. 5. Meilgaard, M., G. V. Civille, and B. T. Carr. 1987. Sensory evalu it as the same or darker was on the average less than 10% ation techniques. CRC Press Inc., Boca Raton, FL. of the end point time for the type I tag and generally about 6. National Food Processors Association. 1988. Safety considerations 20% for the type II tag. For the Life Lines tags (type I), the for new generation refrigerated foods. Refrigerated Foods and Mi visual and instrumental end point difference was less than crobiological Criteria Committee of the National Food Processors Association. Dairy Food San. 8(l):5-7. 3% for Model A20 and less than 3.5% for Model A40 7. Rice, J. 1989. Keeping time/temperature tabs on refrigerated foods. except for one tag. As seen for the I-point tags (type II), Food Proc. August: 149-158. except for one tag, the difference was less than 5%, and 8. Taoukis, P. S., and T. P. Labuza. 1989. Applicability of time- zero for four tags, indicating some tag to tag variability, temperature indicators as shelf life monitors of food products. J. Food Sci. 54:783-788. possibly in the reference printing. These results just show 9. Taoukis, P. S., and T. P. Labuza. 1989. Reliability of time-tempera that there is a good agreement for the end point between ture indicators as food quality monitors under nonisothermal condi instrumental and visual determination. tions. J. Food Sci. 54:789-792. 10. Taoukis, P. S., and T. P. Labuza. 1990. The relationship between CONCLUSION processing and shelf life. pp. 73-106. In G. Birch (ed.), Foods for the 90's. Elsevier Press, London. 11. Wells, J. H., and R. P. Singh. 1988. Application of time-temperature This is the first time that a systematic evaluation of the indicators in monitoring changes in quality attributes of perishable temperature dependence of two types of TTIs intended for and semi-perishable foods. J. Food Sci. 53:148-152,156.

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